1. Field of the Invention
The present invention relates to a method for making a reusable printing plate, a reusing method for the printing plate, and a printing machine. More specifically, the present invention relates to methods for making and reusing a printing plate, and a printing machine capable of writing an image directly on the printing plate based on digital data.
2. Description of Related Art
Recently, in general printing methods, many printing steps have become digitized. That is, image data are digitized by producing an image or text using, for instance, a personal computer, or by scanning an image using a scanner, and a printing plate is formed based directly on the digitized data. In this manner, printing workflow may be improved and the entire printing process may be abbreviated, and achievement of high image quality has been facilitated.
A so-called PS plate (i.e., a presensitized plate) has been generally utilized as a plate in a conventional printing process. The PS plate usually includes a hydrophilic non-image area, which is made of anodized aluminum oxide, and a hydrophobic image area which is formed on the surface of the hydrophilic non-image area by curing a photosensitive resin. However, a plurality of steps is required for making a printing plate using the PS plate, and hence, it is expensive and time-consuming to make such a plate. Accordingly, it is not easy to shorten the time required for the overall printing process and to lower the cost thereof. This is one of the main factors increasing the cost of printed matter, particularly for short run printing. Also, a developing step in which a developer is used, is required for cases where the PS plate is employed, and hence, it is not only a time-consuming process, but also creates a problem, from the viewpoint of preventing environmental pollution, in that developer waste must be treated.
Also, a method, in which a film containing punched out information of an original image is made to contact with a printing plate and is subjected to light exposure, is conventionally used for making the PS plate, and this production of the printing plate is one of the obstacles which prevents the formation of a plate directly from digital data and the digitization of the printing process. Moreover, in a conventional method, a printing plate must be replaced with a new one after the print job, and the used plate is discarded.
There are some commercial methods which, in consideration of the above-mentioned disadvantage of using the PS plate, correspond to the digitized printing process and in which the developing process may be omitted. For instance, Japanese Unexamined Patent Application, First Publication No. Sho 63-102936 discloses a preparation method in which ink containing a photosensitive resin is used for a liquid ink jet printer to be injected onto a printing plate material, and an image area formed by the ink is cured by a photo irradiation process. Also, Japanese Unexamined Patent Application, First Publication No. Hei 11-254633 discloses a method in which a color offset printing plate is made using an ink jet head which discharges a solid ink.
Moreover, a method is known in which a printing plate is made by sequentially applying a laser absorbing layer made of carbon black and a silicone resin layer onto a PET (polyethylene terephthalate) film, and heating the laser absorbing layer by drawing an image using a laser beam so that the silicone resin is subjected to laser ablation. Another method is also known in which a printing plate is made by sequentially applying a lipophilic laser absorbing layer and a hydrophilic layer onto an aluminum plate and subjecting the hydrophilic layer to laser ablation in the same manner as above using the laser beam.
Further, a method has been proposed in which a hydrophilic polymer is used as a recording medium, and a plate is made by converting an irradiated part so as to be lipophilic by optical imaging.
However, by applying the above-mentioned methods, although it may be possible to make a plate directly from digital data, the plate must be replaced with a new one after the print job in order to start the next printing process, and hence, the used plate is also wasted in these methods.
In addition, Japanese Unexamined Patent Application, First Publication No. Hei 10-250027, for instance, discloses a block copy of a latent image using a titanium oxide photocatalyst, a method for forming a block copy of the latent image, and a printing machine including a block copy of the latent image. Also, Japanese Unexamined Patent Application, First Publication No. Hei 11-147360 discloses a method for offset printing using a printing plate containing a photocatalyst. Both of these publications propose methods for reusing a printing plate in which light that activates the photocatalyst, i.e., substantially ultraviolet rays, is used for image writing, and the photocatalyst is converted so as to be hydrophobic via a heat treatment process.
Also, Japanese Unexamined Patent Application, First Publication No. Hei 11-105234 discloses a method for forming a lithographic printing plate in which, after a photocatalyst is activated, i.e., the photocatalyst is converted so as to be hydrophilic by using ultraviolet light, an image area is formed by heat mode drawing.
However, according to Professors Fujishima and Hashimoto of Tokyo University, it is confirmed that titanium dioxide photocatalyst is converted so as to be hydrophilic by a heat treatment process (refer to xe2x80x9cSanka chitan hyoumen no kouzo henka ni tomonau hikarireiki shinsuika gensho no kyodo ni kansuru kenkyuxe2x80x9d which may be translated as xe2x80x9cResearch on the behavior of the photoinduced hydrophilicity phenomenon accompanying structural changes on a titanium dioxide surfacexe2x80x9d, by Minabe et al, published in xe2x80x9cHikari shokubai hanno no saikin no tenkaixe2x80x9d which may be translated as xe2x80x9cRecent developments in photocatalytic reactionsxe2x80x9d, pp. 124-125, (1998) in the Fifth Symposium for Photofunctional Materials Research Association), and hence, it is not possible to make or reuse a printing plate by using the methods disclosed in each of the above publications.
Accordingly, an object of the invention is to provide a method for making a reusable printing plate which may be directly imaged based on digital data and has a high image quality without chemical processing. Another object of the invention is to provide a method for reusing such a printing plate so that the plate may be used repeatedly. Yet another object of the invention is to provide a printing machine capable of using such a printing plate.
The present invention provides a method for making a reusable printing plate in which a hydrophobic image area is formed on at least a part of a hydrophilic surface containing a photocatalyst of the printing plate, comprising the steps of: carrying out a hydrophobic agent application process in which a solution containing an organic compound having a property of reacting with or being fixed to the surface of the printing plate by a heating process and a property of being decomposed by the action of the photocatalyst when irradiated by light having a higher energy than the band gap energy of the photocatalyst is applied as a hydrophobic agent on the surface of the printing plate; carrying out an image area formation process in which at least a part of the surface of the printing plate is subjected to a heating process to form the hydrophobic image area; and carrying out a non-image area formation process in which the organic compound applied to an area other than the hydrophobic image area on the surface of the printing plate is removed.
According to the above method, it is possible to convert the plate surface so as to be hydrophilic by irradiating the plate surface with light having a higher energy than the band gap energy of the photocatalyst. This is due to the action wherein the photocatalyst itself is converted so as to be hydrophilic. The surface converted so as to be hydrophilic functions as a non-image area to which a hydrophobic ink does not attach. A hydrophobic image is formed on the hydrophilic plate surface by applying a solution, which contains an organic compound having a property of reacting with or being fixed to the plate surface via a heating process and a property of being decomposed and removed by the action of the photocatalyst under the irradiation of light having a higher energy than the band gap energy of the photocatalyst, onto the hydrophilic plate surface as a hydrophobic agent, and drying it, if necessary, at a temperature of about room temperature. Although the organic compound contained in the solution is weakly adhered to the hydrophilic plate surface after being applied or dried, once the temperature of the plate is increased to 50xc2x0 C. or more, preferably 100xc2x0 C. or more, the organic compound reacts with or is fixed to the hydrophilic plate surface to form a strong image area.
In accordance with another aspect of the invention, the organic compound is heated by irradiating light having a lower energy than the band gap energy of the photocatalyst so as to react with or be fixed to the surface of the printing plate to form the hydrophobic image area in the image area formation process.
The phrase xe2x80x9clight having a lower energy than the band gap energy of the photocatalystxe2x80x9d means visible rays, infrared rays, etc.; however, from the viewpoint of heating efficiency, the use of infrared rays is preferable.
In yet another aspect of the invention, the surface of the printing plate is washed using a washing liquid in the non-image area formation process.
In yet another aspect of the invention, the organic compound is eliminated by ink tack or cleaning with fountain solution and removed at an initial stage of a printing process in the non-image area formation process.
According to the above method, since the organic compound present on a portion other than the hydrophobic image area, i.e., a non-heating portion, is removed by being washed away or eliminated by ink tack, cleaning with fountain solution, etc., so that the hydrophilic surface of the printing plate is exposed, the plate may function as a printing plate.
Also, by irradiating light having a higher energy than the band gap energy of the photocatalyst onto the plate surface after removing the ink thereon, it becomes possible to decompose the above-mentioned organic compound and return the plate to a state prior to the image formation.
In yet another aspect of the invention, the photocatalyst is a titanium dioxide photocatalyst.
In yet another aspect of the invention, the organic compound is one of an organotitanium compound, an organosilicone compound, a fatty acid dextrin, a thermoplastic resin, and a mixture thereof.
In yet another aspect of the invention, the solution containing the organic compound is an aqueous solution.
The criterion for the xe2x80x9caqueous solutionxe2x80x9d is that the content of the organic compound in the solution when applied is 30 wt. % or smaller.
In yet another aspect of the invention, the solution containing the organic compound is an organic solution.
The criterion for the xe2x80x9corganic solutionxe2x80x9d is that the content of the organic compound in the solution when applied is greater than 30 wt. %.
The present invention also provides a reusing method for a printing plate which is made by using one of the above-mentioned methods, including the steps of: removing ink from the surface of the printing plate after the completion of a printing process; and regenerating the printing plate by converting the surface of the printing plate so as to be hydrophilic by decomposing and removing the hydrophobic image area by the irradiation of light having a higher energy than the band gap energy of the photocatalyst onto the surface of the printing plate.
According to the above reusing method, since the surface of the printing plate is readily regenerated when light having a higher energy than the band gap energy of the photocatalyst is radiated, it is effective for reducing the time and cost required for the regeneration process of the printing plate.
The present invention also provides a reusing method for a printing plate made by using one of the above mentioned methods, including the steps of: removing ink from the surface of the printing plate after the completion of a printing process; and regenerating the printing plate by converting the surface of the printing plate so as to be hydrophilic by alternately carrying out an operation of decomposing and removing the hydrophobic image area by the irradiation of light having a higher energy than the band gap energy of the photocatalyst onto the surface of the printing plate, and an operation of washing the surface of the printing plate by using a washing agent.
According to the above reusing method, since the surface of the printing plate may be more readily regenerated due to synergistic effects of the decomposition action of the photocatalyst and the washing agent when the operation of irradiating light having a higher energy than the band gap energy of the photocatalyst onto the surface of the printing plate and the operation of washing the surface of the printing plate using a washing agent are carried out repeatedly, it is effective for further reducing the regeneration cost.
The present invention also provides a printing machine including: a print drum which is provided with a hydrophilic plate surface containing a photocatalyst; a plate cleaner which removes ink on the plate surface; a hydrophobic agent application device which applies a solution containing an organic compound having a property of reacting with or being fixed to the plate surface via a heating process and a property of being decomposed by the action of the photocatalyst when irradiated by light having a higher energy than the band gap energy of the photocatalyst, as a hydrophobic agent on the plate surface; an image area formation device which forms a hydrophobic image area by subjecting at least a part of the plate surface to a heating process; a dryer which dries the plate surface; and a regeneration device which erases the hydrophobic image area by irradiating light having a higher energy than the band gap energy of the photocatalyst onto the plate surface.
In another aspect of the invention, the above-mentioned printing machine further includes a hydrophobic agent removing unit which removes the organic compound applied on an area other than the hydrophobic image area on the plate surface.
In yet another aspect of the invention, the image area formation device forms an image area by heating the organic compound using irradiation of light having a higher energy than the band gap energy of the photocatalyst so that the organic compound is reacted with or fixed to the plate surface.
In yet another aspect of the invention, the photocatalyst is a titanium dioxide photocatalyst.
In yet another aspect of the invention, the organic compound is one of an organotitanium compound, an organosilicone compound, a fatty acid dextrin, a thermoplastic resin, and a mixture thereof.
According to the above printing machine, a method for making a reusable printing plate, and a reusing method for the printing plate according to the present invention may be suitably carried out using the printing machine.